Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry()

From: Dan Williams <dan.j.williams@intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Barret Rhoden <brho@google.com>
Subject: Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry()
Date: Thu, 4 Oct 2018 18:57:52 -0700	[thread overview]
Message-ID: <CAPcyv4gf5MSaBnzFFrk4XG4OEJFTOqWshk-F==3=4GGMnwNtQw@mail.gmail.com> (raw)
In-Reply-To: <20181004162748.GI28384@quack2.suse.cz>

On Thu, Oct 4, 2018 at 9:27 AM Jan Kara <jack@suse.cz> wrote:
>
> On Thu 27-09-18 11:22:22, Dan Williams wrote:
> > On Thu, Sep 27, 2018 at 6:41 AM Jan Kara <jack@suse.cz> wrote:
> > >
> > > On Thu 27-09-18 06:28:43, Matthew Wilcox wrote:
> > > > On Thu, Sep 27, 2018 at 01:23:32PM +0200, Jan Kara wrote:
> > > > > When dax_lock_mapping_entry() has to sleep to obtain entry lock, it will
> > > > > fail to unlock mapping->i_pages spinlock and thus immediately deadlock
> > > > > against itself when retrying to grab the entry lock again. Fix the
> > > > > problem by unlocking mapping->i_pages before retrying.
> > > >
> > > > It seems weird that xfstests doesn't provoke this ...
> > >
> > > The function currently gets called only from mm/memory-failure.c. And yes,
> > > we are lacking DAX hwpoison error tests in fstests...
> >
> > I have an item on my backlog to port the ndctl unit test that does
> > memory_failure() injection vs ext4 over to fstests. That said I've
> > been investigating a deadlock on ext4 caused by this test. When I saw
> > this patch I hoped it was root cause, but the test is still failing
> > for me. Vishal is able to pass the test on his system, so the failure
> > mode is timing dependent. I'm running this patch on top of -rc5 and
> > still seeing the following deadlock.
>
> I went through the code but I don't see where the problem could be. How can
> I run that test? Is KVM enough or do I need hardware with AEP dimms?

KVM is enough... however, I have found a hack that makes the test pass:

diff --git a/mm/filemap.c b/mm/filemap.c
index 52517f28e6f4..d7f035b1846e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1668,6 +1668,9 @@ unsigned find_get_entries(struct address_space *mapping,
                        goto repeat;
                }
 export:
+               if (iter.index < start)
+                       continue;
+
                indices[ret] = iter.index;
                entries[ret] = page;
                if (++ret == nr_entries)

Is this a radix bug? I would never expect:

    radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start)

...to return entries with an index < start. Without that change above
we loop forever because dax_layout_busy_page() can't make forward
progress. I'll dig into the radix code tomorrow, but maybe someone
else will be me to it overnight.
